EL2386C
250 MHz Triple Current Feedback Amp w/Disable
Applications Information Ð Contd.
creased, bandwidth decreases slightly while sta-
bility increases. Since the loop stability is im-
proving with higher closed-loop gains, it becomes
Capacitance at the Inverting Input
Any manufacturer’s high-speed voltage- or cur-
rent-feedback amplifier can be affected by stray
capacitance at the inverting input. For inverting
gains this parasitic capacitance has little effect
because the inverting input is a virtual ground,
but for non-inverting gains this capacitance (in
conjunction with the feedback and gain resistors)
creates a pole in the feedback path of the amplifi-
er. This pole, if low enough in frequency, has the
same destabilizing effect as a zero in the forward
open-loop response. The use of large value feed-
back and gain resistors further exacerbates the
problem by further lowering the pole frequency.
possible to reduce the value of R below the spec-
F
ified 750X and still retain stability, resulting in
only a slight loss of bandwidth with increased
closed-loop gain.
Supply Voltage Range and Single-
Supply Operation
The EL2386C has been designed to operate with
supply voltages having a span of greater than 3V,
and less than 12V. In practical terms, this means
that the EL2386C will operate on dual supplies
g
g
ranging from 1.5V to 6V. With a single-sup-
ply, the EL2386C will operate from
a
The experienced user with a large amount of PC
board layout experience may find in rare cases
that the EL2386C has less bandwidth than ex-
pected. In this case, the inverting input may have
less parasitic capacitance than expected. The re-
duction of feedback resistor values (or the addi-
tion of a very small amount of external capaci-
tance at the inverting input, e. g. 0.5 pF) will
increase bandwidth as desired. Please see the
3V to
a
12V.
As supply voltages continue to decrease, it be-
comes necessary to provide input and output
voltage ranges that can get as close as possible to
the supply voltages. The EL2386C has an input
voltage range that extends to within 1V of either
a
supply. So, for example, on a single 5V supply,
curves for Frequency Response for Various R
F
and R , and Frequency Response for Various
the EL2386C has an input range which spans
from 1V to 4V. The output range of the EL2386C
is also quite large, extending to within 1V of the
G
C
.
IN-
g
supply rail. On a
5V supply, the output is
b
a
Feedback Resistor Values
The EL2386C has been designed and specified at
therefore capable of swinging from 4V to 4V.
Single-supply output range is even larger because
of the increased negative swing due to the exter-
nal pull-down resistor to ground. On a single
a
a
e
of feedback resistor gives 250 MHz of
gains of 1 and 2 with R
750X. This value
F
b
1 with about 2.5 dB of
3 dB
a
to 4V.
e a
peaking, and 180 MHz of 3 dB bandwidth at
5V supply, output voltage range is about 0.3V
bandwidth at A
V
b
2 with about 0.1 dB of peaking. Since
e a
the EL2386C is a current-feedback amplifier, it is
A
V
Video Performance
also possible to change the value of R to get
F
more bandwidth. As seen in the curve of Fre-
For good video performance, an amplifier is re-
quired to maintain the same output impedance
and the same frequency response as DC levels are
changed at the output. This is especially difficult
when driving a standard video load of 150X, be-
cause of the change in output current with DC
level. Until the EL2386C, good Differential Gain
could only be achieved by running high idle cur-
rents through the output transistors (to reduce
variations in output impedance). These currents
were typically comparable to the entire 3 mA
supply current of each EL2386C amplifier! Spe-
quency Response For Various R and R , band-
G
F
width and peaking can be easily modified by
varying the value of the feedback resistor.
Because the EL2386C is a current-feedback am-
plifier, its gain-bandwidth product is not a con-
stant for different closed-loop gains. This feature
actually allows the EL2386C to maintain about
b
the same 3 dB bandwidth, regardless of closed-
loop gain. However, as closed-loop gain is in-
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